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How do you make foam?

I'm trying to figure out how to make a surfboard without sanding down a foam blank, because blanks are expensive and I'd much rather pour something into a mold shaped like a surfboard. I don't want to have to buy the kits to pour foam because they're also pretty expensive for the scale that I'm talking about. I guess what I'm really asking is how do you make the two liquids that are in the kits to pour your own foam?

Making a surfboard requires a degree sanding and a bit of shaping regardless. Depending on the size of the board you wish to make you can buy a EPS blank for around $60 and figure in shipping. Another avenue you can take is go to your local hardware store and buy a few sheets of rigid insulation foam and a can of Great Stuff and laminate the sheets together to make yourself a blank and then go from there. Good luck!

I don't know anything about surfboards but have made a good many tubes from fiberglass and epoxy resin and have used polyurethane foam to fill various parts.

First, you cannot make the two-part polyurethane foam components, it's WAAAAYYY too expensive. One part is a carefully-formulated di-isocyanate, the other is a carefully-formulated polyol that (usually) has a measured amount of water in it. The diisocyanate reacts with the water to produce CO2 gas that makes the foam. I teach chemistry for a living but would not even attempt to make my own 2-part foam. I buy it....

That being said, to make a large relatively-flat surface like a surfboard, I would consider using the sheets of *extruded* (blue or pink) styrofoam. It's solid foam, not like the "beadboard" that's made of tiny beads. It's quite cheap, about $12 for a 1" sheet 4x8 feet. To make the mild curve I'd "stack" foam near the ends (glue together with a thin layer of epoxy) to build up the thickness, then carve or sand away the excess. rickharris is right, sanding foam is very quick, especially with a power tool. A drum that chucks into a hand drill does a pretty good job. But do it outside as the dust is a real mess!